Focus-grill cathode-ray tube image reproducer modulation with grill voltage

ABSTRACT

Voltage applied to the grill of a focus-grill, color cathode ray tube is modulated at vertical scanning frequency. When scanning along the top and bottom edge portions of the grill, potential applied to the intermediate portion of the grill is increased, thereby correcting for overfocus in the vicinity of the vertical axis by narrowing the width of the focused electron beam on the screen. This avoids color impurity at the top and bottom regions of the screen. When scanning along the region of the horizontal axis, potential applied to the intermediate portion of the grill is decreased, thereby correcting for underfocus in the vicinity of the vertical axis by narrowing the width of the focused beam and thus avoiding color impurity in that region. The voltage modulations are progressively proportioned or damped by resistor means in directions outwardly toward the left and right edge portions of the grill, to which constant potential is applied. Damping the applied voltage modulation to a constant potential avoids color impurity at the edge portions, which have very high color purity sensitivity to grill voltage change.

iinited States Patent [1 1 (Iampbell FOCUS-GRILL CATHODE-RAY TUBE IMAGE REPRODUCER MODULATHON WiTi-I GRILL VOLTAGE [75] Inventor: Francis Joseph Campbell, Yardley,

[73] Assignee: RCA Corporation, New York, NY. [22} Filed: June 28, 1972 [21] Appl. No.: 267,063

[52] US. Cl 315/31 R, 313/92 PD [51] Int. Cl. ..H01j29/70 [58] Field of Search 315/30 R, 30, 31 TV, 31 R,

315/31 A, 15; 313/92 PD; l78/5.4 F

[56] References Cited I UNITED STATES PATENTS 3,416,026 12/1968 Niwa 315/31 TV 3,651,370 3/1972 Goto 11] 3 7903425 51 Feb. 5, ram

[5 7 ABSTRACT Voltage applied to the grill of a focus-grill, color cathode ray tube is modulated at vertical scanning frequency. When scanning along the top and bottom edge portions of the grill, potential applied to the intermediate portion of the grill is increased, thereby correcting for overfocus in the vicinity of the vertical axis by narrowing the width of the focused electron beam on the screen. This avoids color impurity at the top and bottom regions of the screen. When scanning along the region of the horizontal axis, potential applied to the intermediate portion of the grill is decreased, thereby correcting for underfocus in the vicinity of the vertical axis by narrowing the width of the focused beam and thus avoiding color impurity in that region. The voltage modulations are progressively proportioned or damped by resistor means in directions outwardly toward the left and right edge portions of the grill, to which constant potential is applied. Damping the applied voltage modulation to a constant potential avoids color impurity at the edge portions, which have very high color purity sensitivity to grill voltage change.

5 Claims, 6 Drawing Figures PATENTEBFEB sum SNEET'10F3 VOLTAGE 48 SOURCE OSCILLATOR VERTICAL A M i 64 MMAMMM AMA 11mm AM A PATENTED SHEET 2 BF 3 5 ONE VERTICAL SCAN PERIOD TIME- FOCUS-GRILL CATHODE-RAY TUBE IMAGE REPRODUCER MODULATION WITH GRILL VOLTAGE BACKGROUND OF THE INVENTION This invention relates to image reproducers and cathode-ray tubes, and more particularly to focus-grill, color television image reproducers and picture tubes.

A conventional focus-grill, color picture tube comprises a multiple line color phosphor screen, e.g. re peating groups of red, blue and green emitting color phosphor lines or strips, a focus grill of fine grill wires, one for each color group, mounted in spaced relation to the screen and parallel to the phosphor lines, electron gun means for projecting a plurality, e.g., three of electron beams through the grill to the screen, and means for establishing a beam-focusing electric field between the grill and the screen, to focus each beamlet passing between two adjacent grill wires down to approximately the width of each phosphor line.

With the constant grill-screen voltage ratios used in prior focus-grill color picture tubes, there is a problem in that, at optimum voltage ratios, grill focus is poor in certain regions of the grill. Along the top and bottom edge portions of the grill, particularly in the vicinity of the vertical axis, there is overfocus on the screen, and in a region centered at the intersection of the vertical and horizontal axes, there is underfocus. In case of either overfocus or underfocus, the width of the focused beam on the screen is excessive relative to the width of the phosphor strips on the screen, particularly at the high beam currents which exist when the image reproduced is very bright (e.g., a sunlit outdoor scene). The excessive focused-beam width causes color impurity as a result of impingement of the focused-beam upon mutually adjacent phosphor strips fluorescing in different colors. In a region located along the vertical axis about two-thirds distance between the horizontal axis and the top edge portion, and another region located along the vertical axis about two-thirds distance between the horizontal axis and the bottom edge portion, grill focus is satisfactory. These latter regions have been selected as compromises, since designfor perfect focus at the intersection of the horizontal and vertical axes produces intolerably wide, overfocused beam width at the top and bottom edge portions, and design for perfect focus at the top and bottom edge portions produces intolerably wide, underfocused beam width at the intersection of the horizontal and vertical axes.

It can be shown mathematically that the width of the focused beam on the screen is dependent upon the voltage ratio between grill and screen, and so it would appear that modulation of the grill voltage in such a way as to correct for the overfocus at the top and bottom edge portions, and to correct for the underfocus in the region of the intersection of the vertical and horizontal axes, would overcome the problem. However, any attempt to do this encounters a further problem in that variations in grill voltage affect the trajectories of the electrons between grill and screen. The trajectories are so greatly affected at the left and right edge portions of the screen that color purity is adversely affected in these areas.

The foregoing problems are overcome by the present invention. The following detailed description, consid 2 ered in connection with the accompanying drawings, disclose two embodiments of the invention for purposes of illustration only. For definition of the scope of the invention, reference is made to the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. l is an axial sectional top view of a portion of a color television picture tube with associated circuitry embodying principles of the present invention.

FIG. 2 is a transverse sectional back view schematically illustrating grill and screen details of the picture tube of FIG. I, as viewed from the electron gun.

FIG. 3 is an axial sectional top view schematically illustrating a portion of another color television image reproducer embodying principles of the invention.

FIG. 4 is a diagram illustrating the angle of incidence of an electron beam to the grill of FIGS. 1-3.

FIG. 5 is a graph depicting a grill voltage modulation curve.

FIG. 6 is a grill voltage modulation circuit for the picture tube of FIG. 1.

Primed reference characters, where employed, denote elements similar to the elements designated by the corresponding unprimed characters.

DETAILED DESCRIPTION FIG. 1 schematically depicts a portion of a color image reproducer or receiver comprising a cathode-ray tube in the form of a color television picture tube 10 having a logitudinal axis AA. The tube 10 comprises an evacuated glass envelope 12 including an end panel or cap 14 comprising a viewing faceplate 16, a funnel or cone portion 18, and a neck portion 20. A conventional electron gun structure 22 within the neck 20 projects the usual three electron beams through a magnetic deflection yoke 24 which scans the beams in a rectangular raster over a color phosphor screen 26. Preferably, the gun structure 22 is an in-line gun adapted to generate and direct three electron beams along co-planar convergent paths toward the screen 26. As is conventional, the three beams are converged to a common point at or near the screen 26. Line P-P in FIG. ll indicates the plane of deflection of the tube, i.e., the plane in which the axis of each deflected beam, when extended rearwardly, intersects the axis of origin of that beam at zero deflection. The location of the deflection plane along the tube axis changes slightly with changes in the angle of deflection, as is usual in such tubes. The intersection of the deflection plane with the tube axis AA marks the center of deflection of the middle beam of the tube at zero deflection. The screen 26 comprises a multiplicity of parallel lines or strips 28 (FIG. 2) of phosphor fluorescing in different colors. Preferably, the screen is composed of successive trios of red-emitting, green-emitting, and blue-emitting phosphor strips.

A focus grill electrode 30 is positioned adjacent to but spaced from the screen 26. The grill 30 includes a multiplicity of straight, parallel, vertical, elongated wire conductors 32 stretched tautly across the path of the electrons. The wires 32 are preferably held in position by, and embedded in, a fused glass frit seal (not shown) between the funnel portion 18 and the end cap 14 of the envelope 12. As will be developed in detail hereinafter, different electrical potentials are applied to the grill 30 and the screen 26, and the potential difference establishes multiple electrostatic 'lens fields which focus the electron beams passing through the grill onto phosphor strips 28 of the screen 26. The sizes of the grill wires, and of the phosphor strips, have been exaggerated in the drawings for purposes of illustration.

In operation, the yoke 24 scans the electron beams over the grill 30. Stated more specifically, the scanning beams traverse a multiplicity of vertically-spaced, horizontal scan lines to form a raster on the screen. The grill 30 and the screen 26 have a horizontal major axis XX and a vertical minor axis Y-Y. The grill has left and right (as viewed in the drawings) edge portions 34 and 36, respectively, that are located on opposite sides of the vertical axis Y-Y. The grill 30 also includes top and bottom edge portions 38 and 40, that are located on opposite sides of the horizontal axis XX. The grill also has a first intermediate portion 42 that embraces the vertical axis YY and extends from the top edge to the right edge of the grill 30. The left and right edge portions 34 and 36 respectively, include the left and right edges, and extend inwardly from the left and right edges roughly one-third the distance to the vertical axis. The first intermediate portion 42 extends on each side of the vertical axis about one-third the distance from the vertical axis to the left and right edges, respectively. The top and bottom edge portions 38 and 40, respectively include the top and bottom edges and extend inwardly from the top and bottom edges approximately one-third the distance to the horizontal axis. The second intermediate portion 44 extends on each side of the horizontal axis about one-third the distance from the horizontal axis to the top and bottom edges, respectively.

The bottom ends of the grill wires 32 are electrically isolated from each other. The top ends of the wires pass through the envelope and are electrically connected to each other through the structure to be described. Potential is applied to the grill 30 by voltage source means having two components, a variable or modulating grill voltage source 46, and a fixed or constant voltage source 48. The modulating grill voltage source 46 applies varying potential to the intermediate grill portion 42 through a conductor 50. The constant voltage source 48 applies constant potential to the left and right edge portions 34, and 36 of the grill through conductors 52 and 54 respectively.

Constant potential is applied to the screen 26 through a conductor 56 from the constant voltage source 48 that may be a high voltage generator in a conventional television receiver high voltage cage, the low (or zero) voltage terminal of which is connected to the cathodes of the electron gun 22. The potential applied to the screen 26 is higher than the potential applied to the grill 30 in order to produce the desired focusing action on beamlets passing between grill and screen of a focus-grill tube.

The modulating grill voltage source 46 produces a modulating waveform to be described hereinafter. The waveform is produced from a wave derived from a conventional vertical oscillator 62 which drives the vertical deflecting portion of the deflection yoke 24, so the modulating voltage source is operative at vertical scanning frequency. The conductor 63 transmits the wave from the vertical oscillator 62 to the modulating grill voltage source 46. The modulating grill voltage source 46 decreases and increases the potential applied to the intermediate portion 42 of the grill. Decreasing the applied voltage increases the grill-screen potential difference because the screen is at higher potential than the grill. This is effected by the modulating grill voltage source 46 when the electron beams scan the intermediate grill portion 44, i.e., scan the region of the horizontal axis. increasing the difference in potential strengthens the electrostatic lens field between the intermediate portion 42 of the grill and the screen thereby increasing the focusing action of the lens field at the center of the grill, thereby correcting for underfocus in that region and thus narrowing the focused beam width on the screen so that impingement upon phosphor strips of different colors, and resultant color impurity, are avoided. Stated differently, the increase in focusing action narrows the focused beam on the screen, thereby preventing any portion of the focused electron beam from impinging upon two mutually adjacent phosphor strips and thus causing color impurity.

Increasing the voltage applied to the intermediate portion 42 of the grill by modulating voltage source 46 decreases the grill-screen potential difference (because of the higher screen potential), and this is effected by the modulating voltage source when the scanning beams are traversing the top and bottom edge portions 38 and 40 of the grill. Decreasing the difference in potential weakens the lens field between grill and screen, thereby decreasing the focusing action of the lens near the ends of the vertical axis, and thereby correcting for overfocus in those regions and thus narrowing the focused beam width on the screen so that impingement upon phosphor strips of more than one color in a trio and resultant color impurity are avoided.

The problem of excess focused-beam width at the top and bottom edge portions of the grill and in the region of intersection of the vertical and horizontal axes, is limited to the vicinity of the vertical axis (i.e., intermediate portion 42). The problem of excessive focusedbeam width progressively decreases outwardly from the vertical axis to the left and right edge portions 34 and 36, where it is of no significance. However, as previously indicated, the color purity sensitivity of line-grill tubes with respect to grill voltage change is very high at the left and right edge portions because of the effect of-the grill-to-screen electric field on electron trajectories. This latter form of color purity sensitivity progressively decreases from the left and right edges inwardly toward the vertical axis, and is of no consequence in the vicinity of the vertical axis.

In order to overcome the problem of color purity sensitivity to grill voltage change at the left and right edge portions, the grill electrode 30 includes resistor means comprising two resistors 64 and 66 which electrically connect the top ends of the grill wires to one another. The inner ends of resistors 64 and 66 are respectively connected to each other and to the two central grill wires, which are driven at vertical scanning frequency by the modulating grill voltage source 46. The remaining grill wires are successively electrically connected to taps on the resistors in directions outwardly to the left and right edge portions 34 and 36 which are held at fixed potential by the constant voltage source 48. The fixed potential at the edge is chosen to provide a focused beam width which prevents any electron beam focused by the edge portions from impinging upon any two mutually adjacent phosphor strips on the screen and thus causing color impurity along the left and right edge portions of the screen. The resistors 64 and 6d permit the application of a fixed potential to the edge portions 3d and 36 and a modulating potential to the intermediate portion 42 of the grill and allow the remaining grill wires to acquire a continuous potential distribution outwardly toward the left and right edge portions. The resistors may extend not only to left and right edge portions 34 and 36 but also to the outermost wires thereof, as shown in FIGS. 3. and 2. Alternatively, in order to reduce the power input (and resultant heat generation) caused by current flowing in the resistors, and by capacitance of the grill wires, the grill voltage may be fixed at a wire closer to the vertical axis, as in the embodiment of FIG. 3 where the resistors 68 and 70 are shown as terminating at locations spaced inwardly from the left and right edges of the grill. In FIG. 3, each resistor extends outwardly from the center of the grill (i.e., the vertical axis) a distance about onehalf the distance from the center to the respective edge. The remaining outer wires on each side of the grill are tapped to an electrical conductor extending from the resistor to the outer edge of the grill. The structure of FIG. 3 is otherwise similar to the structure of FIG. I.

As has been indicated above, the modulating grill voltage source 46 modulates the grill voltage at vertical scanning frequency in order to reduce the focused electron beam width all along the vertical axis. The required variation of grill voltage to minimize the focused beam width (i.e., to provide the best possible focusing) can be determined for practical purposes by the equation,

V =V /(3 cos 8+1) where: V is the potential applied to the intermediate portion 42 of the grill, V is the potential applied to the screen 26, and 6 is the angle of incidence of the electron beam to the grill measured at the vertical axis. That is, disregarding the rapid horizontal scan motion, 8 is measured between the direction of incidence of the beam and the normal to the grill surface at zero horizontal deflection. The angle 6 is shown in FIG. 4 wherein D is the center of deflection of an electron beam, P is the point where the electron beam intersects the grill plane and p is a point on the same scan line as P with zero deflection. The angle is zero at the intersection of the vertical and horizontal grill axes, and in some tubes will be equal to the vertical deflection angle. However, in other tubes, the presence of a factor such as differing cone and grill potential may bend electron trajectories near the grill and cause 0 to differ slightly from the vertical deflection angle. It will be understood that, as usual in considerations of this nature, the point ofincidence of the electron beams on the grill is considered to be at the center of the geometric pattern defined by the beams on the grill and the direction of incidence is considered to be an average beam direction. Where the beam path is curved, the direction of incidence is considered to be the tangent to the curve at the grill.

The shape of a grill voltage modulation curve during one vertical scan period can be approximated by a parabola, as shown in FIG. 5, where the ratio of grill voltage to screen voltage is the ordinate and time is the abscissa. Correlation of the curve of FIG. with the position of the scanning beams is as follows. When the scanning beams traverse the topmost horizontal scan line, the grill voltage is at its maximum, so the voltage ratio depicted by the curve will be at its maximum. This condition is represented in FIG. 5 at the upper left peak showing a ratio of 36?. As the beams traverse successive scan lines along the top edge portion of the grill, the grill voltage progressively decreases, as represented in FIG. 5, at locations in the upper-left, downsloping portion of the curve. As the scanning beams traverse scan lines progressively closer to the horizontal axis, of the grill, the grill voltage progressively decreases to a nadir when the beam traverses the horizontal axis, and this condition is represented at the lowest point in the curve by a ratio o f 25: Then fas the Beans traverse scan lines progressively approaching the bottom edge portion of the grill, the grill voltage progressively increases, as represented by the upswing of the curve, to again reach its highest value when the scanning beams traverse the bottom edge portion of the grill, reaching a maximum at the lowermost scan line. This completes one vertical scan period, which constitutes one field in interlaced scanning. The beam then traces a second field to complete one frame of the raster, the grill voltage modulation again repeating its swing from left to right along the curve of FIG. 5 as the second field is traced. Then, the first field of the next frame is traced, the grill voltage again being modulated from left to right along the curve of FIG. 5, and so on, as the tube continues to operate.

In a specific example, tube 10 has a 3 by 4 aspect ratio grill with a 25 inch diagonal, and a maximum halfdetlection angle of 55 to each corner. Such a tube is commonly termed a 1 10 tube, and the voltage modulation curve of FIG. 5 is for a tube. Tubes having maximum half-deflection angles other than 55 will have similar curves, differing only in the height of the maxima at the sides of the curve. In any event, in the example, the grill electrode has about 500 wires and the grill wire spacing is about 0.040 inch, measured between centerlines. The grill wires are about 0.002 inch in diameter. The width of the phosphor strips on the screen is about 0.013 inch, and the maximum diameter of each electron beam in the deflection plane is about 0.050 inch. The distance from the deflection center to the grill is about 8.75 inches and the distance from the grill to the screen is about 1 inch. The vertical (frame) scan period isTKITofa second. Screen voltageTs about 25,000 volts, and a fixed potential of about 8330 volts is applied to each of the left and right edge portions of the grill. Each resistor (e.g., 64) is a flat resistor strip having a total resistance of about 2 megohms, the resistance being proportioned uniformly from end to end. The power dissipated in each resistor is about 1 watt.

For such a tube, the grill voltage modulation'given by the above equation requires a voltage swing from about 9150 volts maximum at each side of the curve of FIG. 5, to 6250 volts at the nadir. This corresponds to a V /V variation of from about 0.366 to about 0.250.

The circuitry of modulating grill voltage source 46 can be of any suitable type which produces the waveform of FIG. 5 or a close approximation thereof. It has been observed that the FIG. 5 waveform can be approximated by a parabola, and one form of circuitry for producing such a parabolic waveform is illustrated in FIG. 6. In FIG. 6, vertical oscillator 62 includes a conventional vertical output tube 72 having a cathode which is biased by a grounded resistor 74. A capacitor 76 partially bypasses the resistor 74, and produces a parabolic waveform at vertical scanning frequency. This waveform is transmitted by the conductor 63 to the modulating grill voltage source 46. In the modulating grill voltage source 46, the waveform passes through a variable resistor 78 which may be used to adjust the amplitude of the parabola. The waveform then passes through a coupling capacitor 80 to the grid of a power amplifier tube 82. This grid is connected through the resistor 84 to ground, to establish DC. voltage on the grid. The cathode of the power amplifier tube 82 is biased through a grounded resistor 86, around which is connected a full-bypass capacitor 88.

The waveform is transmitted from the plate of the power amplifier tube 82 through the primary of a voltage step-up transformer 94 the other primary terminal of which is connected to a power supply 13 for tube 82. The modulating grill voltage waveform is produced in the secondary of the transformer 90, and applied to the intermediate portion of the grill through the conductor 50, as described hereinabove. The other secondary terminal of the transformer is connected to a variable voltage source V,, which may be adjusted to set the DC. level of the modulating grill voltage.

Cathode-ray tubes embodying the invention are highly advantageous. The problem of excessive grillfocused beam width in the vicinity of the vertical axis as a result of overfocus at the top and bottom edge portions, and as a result of underfocus in the region of the intersection of the vertical and horizontal axes, is overcome. Further, this solution has been affected without adverse affect upon color purity at the left and right edge portions of the screen, where color purity sensitivity to grill voltage change is very high.

The resistors (e.g., 64) may be of any suitable, conventional resistance material, e.g., carbon. Each resistor may proportion the voltage fluctuations nonuniformly progressively from end to end, instead of uniformly. Alternatively, the frit seal in which the grill wires are embedded may embody a resistance compound and be used to provide the necessary electrical connections and resistance. Grill voltage modulation in accordance with the invention may be employed in double-grill tubes, with the modulating voltage being applied to only one (the lower potential) of the grills. Fewer than three electron beams can be employed, and principles of the invention may be employed in monochrome picture tubes as well as color. Many other modifications of the illustrative embodiments can be made.

I claim:

1. In a cathode-ray tube comprising an evacuated envelope including a phosphor screen having a central vertical axis and a central horizontal axis, an electron gun for projecting an electron beam toward said screen, and a focus grill between said electron gun and said screen, said grill comprising a plurality of spaced apart conductor elements parallel to said vertical axis, the improvement comprising, 7

a. means for distributing a potential applied between said grill and said screen along said focus grill from element to element thereof,

b. first terminal means connected to a center element of said grill and a second terminal means connected to two end elements of said grill,

c. means for applying a modulated electrical potential to said first terminal means,

d. means for applying a first constant electrical potential to said second terminal means and e. means for applying a second constant electrical potential to said screen,

whereby the potential between said grill and said second screen varies from element to element of said grid in the direction of said horizontal axis.

2. In a cathode-ray tube comprising an evacuated envelope including a phosphor screen having a central vertical axis and a central horizontal axis, an electron gun for projecting an electron beam toward said screen, ard a focus grill between said electron gun and said screen for focusing the electron beam onto said screen, said grill comprising a plurality of spaced apart conductor elements parallel to said vertical axis, the improvement comprising,

first terminal means connected to a center element of said grill and second terminal means connected to two end elements of said grill,

resistor means including two resistors for distributing a potential applied between said grill and said screen along said focus grill from element to element thereof, one resistor being connected between said first terminal means and said second terminal means and to a first plurality of said grill elements, and the other resistor being connected between said first terminal means and said second terminal means and to a second plurality of said grill elements.

3. The cathode-ray tube as defined in claim 1 wherein the modulated electrical potential varies in accordance with the equation V V /(3 cos 0 +1) in each side portion of said grill are shorted together. a 

1. In a cathode-ray tube comprising an evacuated envelope including a phosphor screen having a central vertical axis and a central horizontal axis, an electron gun for projecting an electron beam toward said screen, and a focus grill between said electron gun and said screen, said grill comprising a plurality of spaced apart conductor elements parallel to said vertical axis, the improvement comprising, a. means for distributing a potential applied between said grill and said screen along said focus grill from element to element thereof, b. first terminal means connected to a center element of said grill and a second terminal means connected to two end elements of said grill, c. means for applying a modulated electrical potential to said first terminal means, d. means for applying a first constant electrical potential to said second terminal means and e. means for applying a second constant electrical potential to said screen, whereby the potential between said grill and said second screen varies from element to element of said grid in the direction of said horizontal axis.
 2. In a cathode-ray tube comprising an evacuated envelope including a phosphor screen having a central vertical axis and a central horizontal axis, an electron gun for projecting an electron beam toward said screen, and a focus grill between said electron gun and said screen for focusing the electron beam onto said screen, said grill comprising a plurality of spaced apart conductor elements parallel to said vertical axis, the improvement comprising, first terminal means connected to a center element of said grill and second terminal means connected to two end elements of said grill, resistor means including two resistors for distributing a potential applied between said grill and said screen along said focus grill from element to element thereof, one resistor being connected between said first terminal means and said second terminal means and to a first plurality of said grill elements, and the other resistor being connected between said first terminal means and said second terminal means and to a second plurality of said grill elements.
 3. The cathode-ray tube as defined in claim 1 wherein the modulated electrical potential varies in accordance with the equation VG VS/(3 cos2 theta + 1) where VG is the modulated electrical potential, VS is the constant electrical potential applied to the screen, and theta is the angle incidence of the electron beam at said central vertIcal axis with zero deflection along said horizontal axis.
 4. The cathode-ray tube as defined in claim 2 wherein each resistor is connected to less than half of said grill elements.
 5. The cathode-ray tube as defined in claim 4 wherein each resistor is connected to grill elements within a center portion of said focus grill and elements in each side portion of said grill are shorted together. 